1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive including a rotary actuator system for compensating for vibratory components during a seek time of a head of the disk drive.
2. Description of the Related Art
Present 2.5" and 3.5" disk drives (e.g., hard disk drives (HDDs)) are designed to operate in portable and desk-top/server environments, respectively. To reduce cost and weight of a computer system, manufacturers typically fabricate the HDD mounting frame utilizing thin structural members. Therefore, the computer mounting frame is a compliant object prone to vibration. Such a mounting configuration makes the disk drive vulnerable to vibration excited by internal or external sources. More specifically, a HDD with a rotary actuator system is highly sensitive to rotational vibration of its baseplate.
Moreover, a head positioning servo system in an HDD performs several critical tasks.
First, it moves the head to the vicinity of a target in a minimum time using a velocity servo under a seek mode. The head positioning servo system also positions the head on the target track with minimum settle-out time using a position controller without an integrator in it. Finally, the servo system enters the track follow mode with a proportional-integral-derivative-type (PID) position controller.
During a seek mode maximum, rotational acceleration and deceleration torque is imparted by a voice coil motor (VCM)-based actuator. The corresponding reaction torque on the baseplate causes transient rotational vibration that can be detrimental to the positioning accuracy of the read/write heads.
Currently, disk drives have reached 15,000 tracks per inch (TPI), and by year 2000 it is expected to grow above 25,000 TPI. A major obstacle to raising the track density is inadequate head positioning accuracy in the presence of vibration disturbances. Due to exponential growth in TPI, positioning the read/write elements over a track has become a major challenge. Conventional servo control systems require continuous innovations to perform well under increasingly difficult operating conditions.
Furthermore, the mechanical components such as spindle motor assemblies are not perfectly mass-balanced, and during operation they produce harmonic vibration. Harmonic vibration excitation produces both a linear and a rotational oscillatory motion of the entire HDD system. At a 15 kTPI design point, a rotational oscillatory motion of a track with respect to the actuator pivot of about 0.01 thousandth of an inch (i.e., 0.25 micrometer) corresponds to 15% of the track pitch.
When not compensated, a track following error of 15% of track pitch can be detrimental to a disk drive's "soft" and "hard" error rate performance. The positioning error due to this internally produced periodic vibration may be solved using a conventional servo method.
Further, by using a special conventional shock and vibration isolation mount design, the rotational oscillatory components due to internal spindle forces may be minimized.
However, the conventional mount design optimized to decouple internal spindle vibration remains susceptible to external input vibration. By deploying the isolation mounts along a polygon satisfying a particular set of criteria, the externally-generated rotational vibration also can be minimized.
While a unique isolation system could be developed in an attempt to solve vibration problems, it is difficult to commercialize an HDD if each computer manufacturer must achieve a design having low rotational vibrations.
Furthermore, it is noted that using sensors, servo algorithms, and inertial force generators to reduce vibration problem is generally known in the field. However, each application area requires an innovative solution to solve a specific problem.
Thus, as noted above, the seek reaction torque of an HDD acting on its baseplate that is mounted on a weak computer frame can cause a transient vibration. The transient vibration problem arises from several mechanisms.
First, as noted above, transient vibrations arise from a net reaction torque acting on the baseplate causing low frequency (on the order of approximately 100-200 Hz) rigid body oscillations of the baseplate. Secondly, a rotary actuator pivot is coupled to the baseplate casting, and VCM magnets are attached to the baseplate, causing high frequency modal mechanics (2-3 kHz) within the baseplate structure.
The first-mentioned problem has been observed in disk drives with linear actuators, and a feedforward servo solution using a sensor has been proposed for solving such a problem. However, due to the high cost of a reliable and accurate sensor to detect fine motion, a sensor-based solution is considered to be less attractive in low cost HDDs.
The second form of vibration resulting in high frequency modal mechanics is not considered to be a major detractor. However, the high frequency problem can be reduced using a conventional, pure torque generating actuator. An intermediate solution to a pure torque generation has also been proposed. Further, to reduce the severity of reaction force some linear VCM designs have included compliant mounting of the VCM magnets.
Thus, the conventional hard disk drives suffer from serious problems which affect their performance in terms of seek time and the like in a disk drive. Seek time includes two components: move-time and settle-out-time. During a seek, an actuator is driven at high acceleration by an electromagnetically generated torque. Transient dynamics of a hard disk drive (HDD) impact its settle-out time.
Specifically, during a rotational seek motion of a rotary actuator system, a strong reaction torque is applied to a baseplate. The torque is transferred to the baseplate through a magnet housing and actuator pivot assembly. This reaction torque excites the baseplate to vibrate when a disk drive is mounted on a compliant computer structure. The rotational vibration mode of the baseplate in a disk drive having a rotary actuator-based head positioning system causes a transient position error component. The baseplate transient dynamics may last for 20 to 30 ms (or more) following a seek and may impact the settle-out time.
With an ever increasing track density in magnetic disk drives, the reaction-induced vibratory component will degrade a disk drive's access time performance substantially.